With the progress of electronic and electric equipment in recent years, the mounting density of electronic parts has been increased, and new packaging methods have becoming to be used, such as so-called chip scale packages or chip size packages (hereinafter, they will be referred to as CSP) with sizes nearly equal to semiconductor chips, and bare chip packaging.
Reliability is one of the most important characteristics requisite for packaging substrates mounted with various electronic parts, such as semiconductor elements. Particularly, connection reliability against thermal fatigue is very important because it directly affects the equipment containing the packaging substrates.
One of the causes for the lowering of connection reliability is the thermal stress due to the use of various materials having different coefficients of thermal expansion. That is, semiconductor chips have coefficients of thermal expansion of as small as about 4 ppm/° C. while wiring boards for mounting electronic parts have coefficients of thermal expansion of as large as 15 ppm/° C. or more, so that thermal shock results in thermal strain, which results in a thermal stress.
In conventional substrates mounted with semiconductor packages containing lead frames, such as QFP or SOP, the deformation of lead frames absorbs the thermal stress to keep reliability.
In bare chip packaging wherein the electrodes of semiconductor chips and the wiring pads of wiring boards are connected by solder balls or by a conductive paste through small projections referred to as bumps, thermal stress concentrates to the connecting regions, to lower connecting reliability. Putting a resin referred to as “under fill” between chips and wiring boards is known to effectively disperse the thermal stress, but increases packaging steps and cost. Another method is connecting the electrodes of semiconductor chips and the wiring pads of wiring boards by conventional wire bonding, which, however, also increases the packaging steps because wires should be protected by coating a sealing resin.
Because CSP can be mounted together with other electronic parts, various structures have been proposed as disclosed in Surface Mounting Technique, 1997-3, “The future of CSP (fine pitch BGA) being put into practical use”, p 5, Table 1, published by Nikkan Kogyo Shinbunsha. Among them, those containing a tape or carrier substrate as an interconnecting substrate called “interposer” have been increasingly put into practical use. In the above-described table, the structures developed by Tecera Co., Ltd. and TI Co., Ltd. correspond to the above-described structures. Because they contain an interconnecting substrate as an interposer, they excel in connection reliability as published in Shingaku Giho CPM96-121, ICD96-160 (1996-12), “Development of Tape/BGA-type CSP” and Sharp Giho, No. 66 (1996-12), “Chip Size Package Development”.
Between the semiconductor chip and the interconnecting substrate called interposer contained in such a CSP is used an adhesive member that decreases the thermal stress resulting from their difference in coefficient of thermal expansion. The adhesive member requires moisture resistance and high temperature endurance, and there is a demand for film-form adhesive members, which facilitate the production process control.
Adhesives of the film type have been used in flexible printed wiring boards, and most of them contain acrylonitrile butadiene rubber as a main component.
Among those for printed wiring boards that are improved in moisture resistance include an adhesive disclosed in Japanese Patent Application Unexamined Publication No. 60-243180 (1985) which contains an acrylic resin, an epoxy resin a polyisocyanate and an inorganic filler, and an adhesive disclosed in Japanese Patent Application Unexamined Publication No. 61-138680 (1986) which contains an acrylic resin, an epoxy resin, a compound having urethane bonds in molecules and terminated by a primary amine at each end and an inorganic filler.
The adhesive members as described above should release thermal stress and be heat and moisture resistant. In view of production processes, they also should neither allow an adhesive to flow out to electrode areas provided on semiconductor chips for electric signal output nor leave vacant spaces between them and circuits formed on interconnecting substrates. Flowing out of an adhesive to electrode areas causes connection defects of electrodes, and vacant spaces between a circuit and an adhesive tend to deteriorate heat resistance and moisture resistance. It is therefore important to control the flowing amount of adhesives. Further, film-form adhesives containing thermosetting resins are subject to change with passage of time, thereby decreasing the flowing amount and bonding strength. Adhesive members, therefore, require control of the flowing amount and bonding strength throughout their usable periods.
Film-form adhesives containing thermosetting resins gradually cure during storage. The adhesives further cure during various processes for producing a package, including mounting a semiconductor chip on an interconnecting substrate called interposer, fabrication of a package, etc. It is preferable to use an adhesive having a longer usable period to improve the processability of the adhesive and the connection reliability of semiconductor chips. That is, the longer the usable period is, the less the flowing amount and bonding strength decrease due to the change with passage of time, facilitating the control of the flowing amount and bonding strength.
The usable periods of conventional film-form adhesives could be increased by decreasing the content of the curing accelerator in an adhesive composition, but the curing rate was problematically lowered on curing the adhesives to cause foaming. There has been a demand for adhesives, which do not foam but have longer usable periods and as well satisfy the requirement for low elasticity, heat resistance and moisture resistance.
Further, adhesives for use in semiconductor packages or wiring generally contain thermosetting high molecular weight components such as epoxy resins to improve heat resistance. The thermosetting high molecular weight components, however, have the defects of requiring a high temperature and a long time for curing. To solve the defect, curing accelerators have been used together with the thermosetting resins. Blending a curing accelerator greatly improves the curability, but has caused another problem that the reaction proceeds even at room temperature, thereby changing the flowability of the adhesive during storage at room temperature to make the adhesive commercially useless. To solve the new problem, it was proposed to use a latent curing accelerator having no activity at room temperature. For example, Japanese Patent Application Unexamined Publication No. 9-302313 (1997) discloses the use of a very latent imidazole as a curing accelerator for epoxy resins in adhesive compositions. The latent curing accelerator improves storage stability. However, it has been found that because the production process of adhesive films includes a heat treatment step for curing the adhesive composition to B-stage and the partially reacted latent curing accelerator becomes active even at room temperature, the reaction gradually proceeds to deteriorate storage stability. This has caused a demand for further improvement of storage stability.